Integral Versus Discrete Panel SPDs: A Comparison

Meaningful purchase decisions usually include a pro versus con or benefit versus cost analysis. The intent is to ensure that capital equipment decisions are in the best interest of a corporation for the long term. Let's use this same process to determine whether you should buy service equipment and panelboards with integral surge protection devices (SPDs) or buy individual SPDs in separate discrete

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Meaningful purchase decisions usually include a pro versus con or benefit versus cost analysis. The intent is to ensure that capital equipment decisions are in the best interest of a corporation for the long term.

Let's use this same process to determine whether you should buy service equipment and panelboards with integral surge protection devices (SPDs) or buy individual SPDs in separate discrete panels and attach them to the load side of a disconnecting means.

An SPD mounted internal to service equipment and panelboards connects directly to the energized bus. An SPD mounted in a separate enclosure is connected (wired) through the service equipment and panelboards to the load side of a dedicated breaker (i.e. 30A, 3-pole circuit breaker).

Arguments For Integral SPDs

There are several advantages of integrating an SPD into service equipment. The one obvious advantage is the convenience of purchasing a single integrated unit to accomplish two engineering requirements. This advantage is helpful in creating the purchase order because it requires less paperwork and follow-up. It also makes the installation easier because it is now a one-step process.

Another advantage to integral surge protection is there is minimal wire length due to the fact that the input of the SPD is typically connected directly to the services entrance conductors. The result is lower overall let-through voltage, also known as clamping.

Arguments Against Integral SPDs

Required maintenance

This may present a problem. Manufacturers design service entrance enclosures to enclose the service conductor and other energized parts and guard against accidental contact. By definition, servicing an integrated SPD causes a forced access into energized parts within the service entrance equipment.

We sometimes overlook the associated liability concerns related to this access. Facilities personnel are not always experienced with service entrance equipment maintenance including their safety-related issues. It is fairly common to meet electricians whose work experience is primarily related to low-voltage electrical equipment or HVAC equipment. Given this expertise, they generally have respect for electrical service equipment, but may not be prepared to maintain it.

The 2002 NEC Handbook addresses this issue by stating certain provisions to the intent of Art. 408:

“Where it can be demonstrated that it is unfeasible to disconnect or de-energize the service conductors supplying a service switchboard, qualified electricians may be required to work on these switchboards with the load terminals de-energized but with the service bus energized. Barriers are required in all service switchboards to proved physical separation (adequate distance or obstacle) between load terminals and the service busbars and terminals, thus providing some measure of safety against inadvertent contact with line-energized parts during maintenance and installation of new feeders or branch circuits. In most multisection switchboards, barriers are not required, because the line-side conductors and busbars are not in the same switchboard sections that contain the load terminals. It must be clearly understood that de-energizing the load side of a switchboard, by operation of a disconnecting means, does not de-energize the ungrounded service conductors. Every effort should be made to make arrangements to completely disconnect power from the equipment before performing and work inside. For complete disconnection that is not feasible, the installer should become familiar with NFPA 70E, Electrical Safety Requirements for Employee Workplaces. This industry recognized document provides the guidance for protective equipment and appropriate work rules that must be followed for working on energized equipment.”

A conventional SPD installed in a separate panel on the load side of a dedicated service disconnect reduces this liability concern. You can do maintenance on a dedicated SPD panel by de-energizing the circuit breaker disconnect for the SPD and performing the necessary cleaning or repair on a safe and separate de-energized panel. The Figure, right, shows an example of an SPD installation with a discrete panel SPD.

Space limitations

This relates to the space within a service entrance panel. There may not be enough room within the service entrance to adequately house an SPD and the essential service disconnecting means.

“The service disconnecting means shall be installed at a readily accessible location either outside of the building or structure or inside nearest the point of entrance of the service conductors.”

The key to this statement is the term readily accessible. When an SPD is installed within service equipment this “readily accessible” term for the disconnecting means may become compromised. Service entrance cabinets and panelboards are designed to house switchgear and their energized input and output conductors. When the same cabinet is used to house an SPD with its own input conductors, the available space may become limited. This may not be significant, but why take the chance when alternatives exist.

Combustible materials

The 2002 NEC Handbook, under Sec. 230.70 General (A) Location notes:

“The length of service-entrance conductors should be kept to a minimum inside buildings, since power utilities provide limited over current protection and, in the event of a fault, the service conductors could ignite nearby combustible material.”

Metal oxide varistors (MOVs), a common component in SPD products, consist of combustible materials. During a lightning strike, a significant amount of energy can cause the MOVs to turn on and conduct current. If the SPD is not sized appropriately to accommodate the energy with a particular lightning strike, an event similar to a fault current condition could cause a combustible event. Even in cases where the combustible event does not occur, a large surge event may cause the SPD to emit smoke. The service could become contaminated, requiring a costly replacement.

Lower clamping due to shorter lead length

After carefully reviewing the described advantage of lower clamping due to shorter lead length, we noted that within a typical service entrance and panelboard enclosure, the direct connection from the integral SPD to the energized conductors and the neutral is approximately 20 in. In most applications, you can mount the panel housing the discrete SPD so the total length of energized conductors is about the same.

Service entrance equipment protection

SPD products are specifically designed to protect sensitive down line electronics from power surges. The correct installation of a discrete SPD panel requires a separate service entrance with dedicated disconnecting means to complete the infrastructure of the electrical system. In a discrete SPD installation, the down line load does not require the SPD to complete the network. So, in the case of a fault current condition or MOV failure, the dedicated disconnect would open, and there would be no damage to the service entrance equipment.

If the same fault current condition or MOV failure occurred in a service entrance with integral SPD, the NFPA standards recommend replacement of the service entrance equipment.

Circuit breaker advantages

A dedicated circuit breaker mounted ahead of a discrete SPD provides many advantages. This circuit protection device is safe and inherently prevents exposure to its live parts by its specifically designed isolation characteristics. A circuit breaker, also known as an overcurrent protective device (OPD) is designed to open on an over current condition (i.e. swell or temporary overvoltage) as well as a fault current condition.

Another benefit of the circuit breaker is that it can also serve as a disconnect for the down line SPD. A circuit breaker is resettable and, if properly selected, has current limiting characteristics that are beneficial to the functionality of a well-designed and implemented SPD product.

Normally, you select and coordinate the protective device to give the load-side cable sufficient short-circuit protection. You can do this by plotting the time-current curves of the wire and protective device on the same log-log graph paper or by using commercially available engineering software. You must do this first before applying the appropriate overload protection of the wire, based on the rating of the wire and its load characteristics.

Entrance location of service entrance conductors

An integral SPD may cause limitations in locating where the service entrance conductors actually enter the equipment back box. The electrical contractor may not have the flexibility of alternate back box entrances because of the location of the integral SPD. In a discrete SPD panel, the contractor can feed the service conductor from any location around the panel, which includes the top, bottom or either side.

Inductive heating effects

This concern relates to an integral SPD. The NEC Sec. 408.3(B) (Overheating and Inductive Effects) states:

“The arrangement of busbars and conductors shall be such as to avoid overheating due to inductive effects.”

If an SPD is integrated into a switchboard or panelboard, you probably should have a thermal analysis performed to ensure there is no inductive heating or hot spots.

Summary

After reviewing the issues related to integral and discrete SPDs, it appears the discrete SPD may be the safest and most functionally effective method of providing comprehensive surge protection. Incorporating an integral SPD into service entrance equipment has initial installation benefits, but these may not provide a cost-effective solution for the long-term use of the product. Oftentimes, the easy way out in a purchase decision can have long-term ramifications and costly consequences. The decision is yours.

Kenneth Brown is director of engineering power quality, Leviton Mfg. Companies, San Diego. You can reach him atKBrown@leviton.com.

Important NEC Definition

The NEC defines “service equipment” as: “The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s), and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and cutoff of the supply.”

Service equipment may consist of circuit breakers or fused switches provided to disconnect all ungrounded conductors in a building or other structure from the service-entrance conductors. The disconnecting means at any one location is not allowed to consist of more than six circuit breakers or six switches and is required to be readily accessible either outside the building or structure or inside nearest the point of entrance of the service-entrance conductors.

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